A messy chestnut refers to the act of a man pulling his erect penis out of a woman's vagina and ejaculating on the general chest area of the female. The man then sucks and inhales the semen into his own mouth until he collects it all. The woman opens her mouth and allows the man to spit the semen into her mouth and then let's it drip off of her chin and run back down her chest area.
I gave her a messy chestnut